http://php.net/manual/en/language.variables.scope.php
This can cause some problems in that people may inadvertently change a global variable.
這是為了避免無意間改變到全域變數。
<?php $a=123; /* global scope 全域範圍 */ function test() { echo $a; /* reference to local scope variable so output nothing */ echo $GLOBALS['a']; // 123 global $a; echo $a; // 123 $a = 456; echo $a; // 456 } test(); ?>